Semiconductor lasers are widely used in applications such as telecommunications and optical data storage. However, the threshold current of conventional laser depends upon the temperature at which the laser is operated, and this temperature dependence causes variability in the laser's optical output even if the driving current is constant. Correction of the variability of the optical output can require complicated and costly measures such as cooling systems and feedback loops. It would therefore be preferable for the threshold current of a laser to be temperature-independent. In some cases, quantum dot (QD) lasers have demonstrated temperature insensitivity superior to that of quantum well (QW) lasers. However, conventional QD lasers still exhibit significant temperature sensitivity.
The threshold current in a semiconductor laser is the lowest injection current at which lasing emission occurs. At the lasing threshold, the optical gain of the active medium becomes equal to the total losses—the total losses being equal to the sum of the mirror losses and the internal losses. A major source of temperature dependence of threshold current in QD lasers is parasitic recombination of carriers—i.e. electrons and holes—outside the QDs. Such recombination occurs primarily in the optical confinement layer (OCL) of the device. In most conventional QD lasers, the OCL is a conductive material in which the QDs are embedded. For example, FIG. illustrates a prior art structure 502 which includes n-type and p-type cladding layers 504 and 512, and an OCL 514 comprising first and second OCL portions 506 and 510. Self-organized QDs 508 are embedded between the first and second OCL portions 506 and 510. The current flowing through the device not only includes current IQD resulting from carriers entering the QDs 508 and recombining to generate useful photons, but also includes parasitic current resulting from recombination of carriers in the OCL 514. The amount of this parasitic current depends on the rate at which the carriers recombine in the OCL 514, which is proportional to the populations of electrons and holes in the OCL 514. As is well-known in the art, the ratio of the population of electrons in the OCL 514 to the population of electrons in the QDs 508 increases with temperature. Similarly, the ratio of the population of holes in the OCL 514 to the population of holes in the QDs 508 also increases with temperature. As a result, the component of threshold current density associated with recombination in the OCL 514 increases with temperature, thereby causing the total threshold current also to increase with temperature.
An additional source of temperature-sensitivity in QD lasers is non-uniformity of the sizes of the QDs 508. In a typical QD laser, the QDs tend to exhibit significant size variation. The QD size variation causes undesired pumping of non-lasing QDs, an effect which further contributes to the temperature-dependence of the threshold current of the device.
Yet another cause of temperature-sensitivity in QD lasers is recombination from non-lasing (typically higher-energy) carrier states in the quantum dots. If a QD has electron and hole states other than the states being used for lasing, the extra states can be populated by thermally-excited carriers, an effect which is temperature-dependent. The carriers in the extra states can recombine to generate parasitic current. This thermally activated parasitic current adds to the temperature-dependence of the threshold current of the device.
Still another source of temperature-sensitivity in QD lasers is the violation of charge neutrality in individual quantum dots. The optical gain of a QD laser is A=K1(Fn+Fp−1), where K1 is a constant, Fn is the probability of occupancy of the lasing electron state in a QD, and Fp is the probability of occupancy of the lasing hole state in a QD. The current associated with carrier recombination in the QDs is IQD=K2FnFp, where K2 is a constant. If a QD is charge-neutral—i.e., if the number of electrons equals the number of holes—then Fn=Fp, and therefore, A=K1(2Fn−1) and IQD=K2Fn2. The amount of gain A required to reach the lasing threshold is independent of temperature, and therefore, the value of Fn required to reach the lasing threshold is also independent of temperature. Because IQD is a function of Fn, the threshold value of IQD is similarly temperature-independent. However, if the above condition of charge neutrality is violated in a QD—i.e., if there are one or more extra electrons, or one or more extra holes—then Fn and Fp not only tend to be unequal, but as is well-known in the art, Fn and Fp typically vary differently from each other as functions of temperature. See L. V. Asryan and R. A. Suris, “Charge Neutrality Violation in Quantum-Dot Lasers,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 2, April 1997. As a result, although the threshold value of A=K1(Fn+Fp−1) does not depend on temperature, the threshold value of IQD=K2FnFp can—and in fact, typically is—temperature-dependent. Therefore, if charge neutrality is violated, the total threshold current is typically temperature-dependent. Violation of charge neutrality is the dominant cause of temperature sensitivity at low temperatures.